We study dynamical generation of entanglement in bipartite quantum systems, characterized by purity (or linear entropy), and caused by the coupling between the two subsystems. The explicit semiclassical theory of purity decay is derived for integrable classical dynamics of the uncoupled system and for localized (general Gaussian wave packet) initial states. Purity decays as an algebraic function of (time) $\times$ (strength of perturbation), independently of the Planck’s constant.

We show that a perfect quantum-state transmission can be realized through a spin chain possessing the commensurate structure of an energy spectrum, which is matched with the corresponding parity. As an exposition of the mirror inversion symmetry discovered by Albanese et al. (e-print quant-ph∕0405029), the parity matched commensurability of the energy spectra helps us to present preengineered spin systems for quantum information transmission. Based on these theoretical analyses, we propose a protocol of near-perfect quantum-state transfer by using a ferromagnetic Heisenberg chain with uniform coupling constant, but an external parabolic magnetic field. The numerical results show that the initial Gaussian wave packet in this system with optimal field distribution can be reshaped near perfectly over a longer distance.

We report measurements of the lifetimes of the $3d{}^{2}D_{5∕2}$ and $3d{}^{2}D_{3∕2}$ metastable states of a single laser-cooled ${}^{40}\mathrm{Ca}^{+}$ ion in a linear Paul trap. We introduce a measurement technique based on high-efficiency quantum state detection after coherent excitation to the $D_{5∕2}$ state or incoherent shelving in the $D_{3∕2}$ state, and subsequent free, unperturbed spontaneous decay. The result for the natural lifetime of the $D_{5∕2}$ state of 1168(9) ms agrees excellently with the most precise published value. The lifetime of the $D_{3∕2}$ state is measured with a single ion and yields 1176(11) ms which improves the statistical uncertainty of previous results by a factor of four. We compare these experimental lifetimes to high-precision ab initio all order calculations [$D_{3∕2}$ state: 1196(11) ms; $D_{5∕2}$ state: 1165(11) ms] and find a very good agreement. These calculations represent an excellent test of high-precision atomic theory and will serve as a benchmark for the study of parity nonconservation in ${\mathrm{Ba}}^{+}$ which has similar atomic structure.

Recent low energy $(e,2e)$ experimental results on calcium in doubly symmetric kinematics are analyzed by using distorted-wave Born approximation theory. The final state electron-electron correlation is included through angle-dependent effective charges and exchange distortion in semiclassical local approximation. It is found that the present results are able to qualitatively reproduce most features of the recently reported experimental data.

The complete set of second-, third-, and fourth-order van der Waals coefficients $C_n$ up to $n=32$ for the $\mathrm{H}\left(1s\right)\text{−}\mathrm{H}\left(1s\right)$ dimer have been determined. They are computed by diagonalizing the nonrelativistic Hamiltonian for hydrogen to obtain a set of pseudostates that are used to evaluate the appropriate sum rules. A study of the convergence pattern for $n⩽16$ indicates that all the $C_{n⩽16}$ coefficients are accurate to 13 significant digits. The relative size of the fourth-order $C_n^{\left(4\right)}$ to the second-order $C_n^{\left(2\right)}$ coefficients is seen to increase as $n$ increases and at $n=32$ the fourth-order term is actually larger.

The entropy-temperature curves are calculated for noninteracting fermions in a three-dimensional optical lattice. These curves facilitate understanding of how adiabatic changes in the lattice depth affect the temperature, and we demonstrate regimes in which the atomic sample can be significantly heated or cooled. When the Fermi energy of the system is near the location of a band gap, the cooling regimes disappear for all temperatures and the system can be heated by only lattice loading. For samples with greater than one fermion per site, we find that lattice loading can lead to a large increase in the degeneracy of the system. We study the scaling properties of the system in the degenerate regimes and assess the effects of nonadiabatic loading.

The factorial moments of photon counting statistics from a single molecule coupled to a quantum bath are expressed in terms of multipoint quantum correlation functions and represented by double-sided Feynman diagrams, in close formal analogy with nonlinear spectroscopy. At infinite temperature we recover the results of stochastic models of spectral diffusion where the bath dynamics is independent on the state of the system and the moments are described by lower-order correlation functions.

In a Kerr nonlinear blackbody, bare photons with opposite wave vectors and helicities are bound into pairs and unpaired photons are transformed into a different kind of quasiparticle, the nonpolariton. The nonpolariton system constitutes free thermal radiation in the blackbody. The present paper investigates the radiation properties of a Kerr nonlinear blackbody. We found that the spectral energy density and radiation pressure of a Kerr nonlinear blackbody are larger than those of a normal blackbody and that these two quantities are monotonically decreasing functions of the Kerr nonlinear coefficient. Above the transition temperature the photon system is in a normal thermal radiation state, but below the transition temperature it is in a squeezed thermal radiation state. In the transition from the normal to the squeezed thermal radiation state, the phase symmetry of the photon system is spontaneously broken.

One of the main advantages of an optical approach to quantum computing is the fact that optical fibers can be used to connect the logic and memory devices to form useful circuits, in analogy with the wires of a conventional computer. Here we describe an experimental demonstration of a simple quantum circuit of that kind in which two probabilistic exclusive-OR (XOR) logic gates were combined to calculate the parity of three input qubits.

We propose a scheme to generate a four-photon polarization-entangled cluster state by using only linear optical elements and four-photon coincidence detection. The scheme requires two single-photon states and one two-photon polarization entangled state as input resources. Then, we consider the realistic implementation of the scheme by using the spontaneous parametric down-conversion as photon resources. It is shown that under certain conditions, the four-photon polarization cluster state can be generated with a high fidelity, and the scheme is feasible by current experimental technology.

The low-energy spectra of the single-mode $h\otimes (G+H)$ linear Jahn-Teller model is studied by means of exact diagonalization. Both eigenenergies and photoemission spectral intensities are computed. These spectra are useful to understand the vibronic dynamics of icosahedral clusters with partly filled orbital quintet molecular shells, for example ${\mathrm{C}}_{60}$ positive ions.

We have investigated collisions between transversely polarized electrons and Ar, in which the Ar is simultaneously ionized and excited to the ${\mathrm{Ar}}^{+*}\left[3p^4\left({}^{1}D\right)4p\right]$ states. The Stokes parameters of the fluorescence emitted in the following transitions was measured: $\left({}^{1}D\right)4s{}^{2}D_{5∕2}-\left({}^{1}D\right)4p{}^{2}F_{7∕2}$ $\left(461.0\mathrm{nm}\right)$, $\left({}^{1}D\right)4s{}^{2}D_{5∕2}-\left({}^{1}D\right)4p{}^{2}F_{5∕2}$ $\left(463.7\mathrm{nm}\right)$, $\left({}^{1}P\right)3d{}^{2}D_{5∕2}-\left({}^{1}D\right)4p{}^{2}D_{5∕2}$ $\left(448.2\mathrm{nm}\right)$, and $\left({}^{1}D\right)4s{}^{2}D_{3∕2}-\left({}^{1}D\right)4p{}^{2}P_{3∕2}$ $\left(423.7\mathrm{nm}\right)$. We develop the angular momentum algebra necessary to extract from these data, starting from the overall atomic $J$ multipoles, the partitioning of orbital angular momentum into the ${}^{1}D$ core electric quadrupole and hexadecapole moments, and the outer $4p$ electric quadrupole moment. The magnetic dipole of the outer electron is also determined. This procedure requires the assumption of good $LS$ coupling for these states, which is justified. We recouple these individual core- and outer-electron moments to calculate the initial electric quadrupoles, hexadecapoles, and hexacontatetrapoles of the initial excited-state manifold. The detailed time structure of the electron-atom collision is considered, as well as the time evolution of the excited ionic state. The Rubin-Bederson hypothesis is thus shown to hold for the initial ionic $L$ and $S$ terms. The consequences of the breakdown of $LS$ coupling are considered. From the circular polarization data, estimates of the relative importance of direct and exchange excitation cross section are made. We discuss experimental issues related to background contributions, Hanle depolarization of the fluorescence signal, and cascade contributions. Nonlinearity of the equations relating the Stokes parameters to the subshell multipole moments complicates the data analysis. Details of the Monte Carlo terrain-search algorithm used to extract multipole data is discussed, and the implications of correlation between the various subshell multipole moments is analyzed. The physical significance of the higher-order multipole moments is discussed, and graphical representations of the effects of these multipoles on the excited ionic charge clouds is presented.

We studied the photoelectron spectra of He atoms resulting from double photoexcitation to autoionizing states by an attosecond xuv pulse in the presence of intense few-cycle Ti:sapphire lasers using the time-dependent hyperspherical close-coupling method. In its first application we show that the combination of a weak xuv pulse and an intense infrared laser offers an efficient means for probing states that cannot be reached by single photoabsorption experiments alone. The method provides an alternative approach of studying Stark induced states at field strength much higher than that available for a dc electric field. Using parameters from the presently available attosecond pulses and infrared lasers we showed that the four singlet $2\ell 2{\ell }^{\prime }$ doubly excited states of He are prominently excited in such experiments. The dependence of the shape of the autoionizing states thus generated has been studied with respect to the intensity and the carrier-envelope phase of the few-cycle laser pulses.

The energy and structure of a dilute hard-disks Bose gas are studied in the framework of a variational many-body approach based on a Jastrow correlated ground-state wave function. The asymptotic behaviors of the radial distribution function and the one-body density matrix are analyzed after solving the Euler equation obtained by a free minimization of the hypernetted chain energy functional. Our results show important deviations from those of the available low density expansions, already at gas parameter values $x\sim 0.001$. The condensate fraction in 2D is also computed and found generally lower than the 3D one at the same $x$.

In this Brief Report the properties of both dynamical and static entanglement in the integrable quantum dimer are studied in terms of the reduced-density linear entropy and von Neumann entropy with various coupling parameters, total boson numbers, and initial states. The mean entanglement, which is defined to be averaged over time, is used to describe the influence of the classical separatrix on the behavior of entanglement. It is shown that the mean entanglement exhibits a maximum near the position of the corresponding classical separatrix energy and that the static entanglement of the state with the largest eigenvalue of the quantum spectrum displays a maximum near the bifurcation point. For weak coupling and larger total boson number the maximum entanglement state is exactly at the position of the classical separatrix and bifurcation. In strong coupling all initial states have nearly the same mean entanglement.

The stabilization of entanglement caused by action of a classical driving field in the system of two-level atoms with the dipole interaction accompanied by spontaneous emission is discussed. An exact solution shows that the maximum amount of concurrence that can be achieved in the Lamb-Dicke limit is 0.43. Dependence of entanglement on interatomic distance and the classical driving field, beyond the Lamb-Dicke limit, is examined numerically.

An experimentally feasible scheme for implementing quantum dense coding in cavity QED is proposed. In the scheme the atoms interact simultaneously with a highly detuned cavity mode with the assistance of a classical field. The scheme is insensitive to the cavity decay and the thermal field. Bell states can be exactly distinguished via detecting the atomic state, and the quantum dense coding can be realized in a simple way.

We examine the execution of general $\mathrm{U}\left(1\right)$ transformations on programmable quantum processors. We show that, with only the minimal assumption of availability of copies of the $1$-qubit program state, the apparent advantage of existing schemes proposed by G. Vidal et al. [Phys. Rev. Lett. 88, 047905 (2002)] and M. Hillery et al. [Phys. Rev. A 65, 022301 (2003)] to execute a general $\mathrm{U}\left(1\right)$ transformation with greater probability using complex program states appears not to hold.

The multiconfiguration Dirac-Hartree-Fock (MCDHF) model has been employed to calculate the atomic expectation values responsible for the hyperfine splittings of the $7s7p{}^{3}P_{1,2}$ and ${}^{1}P_1$ levels of radium. Calculated electric field gradients, together with the experimental electric quadrupole hyperfine structure constants, allow us to extract a nuclear electric quadrupole moment $Q\left({}^{223}\mathrm{Ra}\right)$ of 1.21(0.03) barn. This value is in good agreement with the semiempirical determination based on neutral radium hyperfine and fine structure, but differs from the latest result from an alkali-like radium ion.

We investigate a process by which an optical lattice potential can transfer two unbound ultracold atoms in a lattice site to an untrapped dimer. The transition is mediated by the anharmonic terms in the lattice potential, which couple the relative and center-of-mass coordinates of the two atoms. We calculate the rate of decay from the ground state in a lattice cell to the last bound dimer state as a function of the -wave scattering length and optical lattice depth. The decay rate has a maximum value of , where is the atomic recoil energy. Consequently typical molecular production rates are much smaller than . Explicit results are given for atoms tuned near a Feshbach resonance.